Hydroconversion of Methyl Laurate as a Model Compound to

Article pubs.acs.org/EF

Hydroconversion of Methyl Laurate as a Model Compound to Hydrocarbons on Bifunctional Ni2P/SAPO-11: Simultaneous Comparison with the Performance of Ni/SAPO-11 Sha Zhao,† Mingfeng Li,‡ Yang Chu,‡ and Jixiang Chen*,† †

Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China ‡ Research Institute of Petroleum Processing, SINOPEC, 18 Xue Yuan Road, 100083 Beijing, China S Supporting Information *

ABSTRACT: The bifunctional Ni2P/SAPO-11 was tested for the hydroconversion (involving deoxygenation and hydroisomerization) of methyl laurate as a model compound to hydrocarbons. The influences of reaction conditions, catalyst stability, and catalyst deactivation were investigated. For comparison, the performance of Ni/SAPO-11 was also examined. The result shows that the increase of temperature and the deceases of weight hourly space velocity (WHSV) and H2 pressure favored the conversion of methyl laurate meanwhile promoted the decarbonylation and hydroisomerization as well as cracking reactions. Apart from the Ni sites that were dominating for deoxygenation, the acid sites also affected the deoxygenation pathway. Due to more medium strength acid sites, Ni/SAPO-11 gave higher selectivity to isoalkanes and more preferentially catalyzed the hydrodeoxygenation pathway to produce the C12 hydrocarbons than Ni2P/SAPO-11. During the test for 101 h, Ni2P/SAPO-11 exhibited greatly superior stability to Ni/SAPO-11 for the deoxygenation of methyl laurate, while both Ni2P/SAPO-11 and Ni/ SAPO-11 were deactivated for the hydroisomerization. Under the condition of 360 °C, 3.0 MPa, WHSV of 2 h−1, and H2/methyl laurate molar ratio of 25, the conversion of methyl laurate was close to 100% and the total selectivity to isoundecane and isododecane decreased from 36.9% to 28.6% on Ni2P/SAPO-11. To explore the catalyst deactivation, the fresh and the used catalysts were characterized by X-ray diffraction, transmission electron microscopy, scanning electron microscopy, thermogravimetric analysis, Raman spectroscopy, and N2 adsorption−desorption. The sintering of Ni particles and carbonaceous deposit contribute to inferior stability of Ni/SAPO-11 for both deoxygenation and hydroisomerization, while no obvious sintering of Ni2P particles took place and the carbonaceous deposit mainly led to the loss of the activity for hydroisomerization on Ni2P/SAPO-11. We propose that carbonaceous deposit mostly formed on the acid sites that are indispensible for hydroisomerization.

1. INTRODUCTION Fossil fuels play an important role in human progress and social development. However, their reserves are being depleted and their combustion emissions lead to severe environmental pollution.1 As a result, renewable energies have attracted great attention. Among them, biodiesel (FAME), produced by the transesterification of vegetable oils and methanol, is very environment-friendly.2 It not only meets the demand for energy but also reduces the pollution to the environment. However, biodiesel has some disadvantages such as poor chemical stability and low energy density because of its high oxygen content.3,4 Removing oxygen from vegetable oils via hydrodeoxygenation (HDO), decarbonylation, and decarboxylation is alternative to producing diesel-like hydrocarbons,5,6 which is superior to biodiesel in terms of viscosity and oxidation stability.7 Nevertheless, diesel-like hydrocarbons mainly consist of n-alkanes with high pour point and high cloud point, which lead to a poor low-temperature performance. To solve this problem, the hydroisomerization of n-alkanes is often adopted on the bifunctional catalysts. So far, two-step and one-step processes have been used to produce the diesel with good low temperature fluidity. In the two-step process,8 vegetable oil is first deoxygenated on a commercial sulfide NiMo/γ-Al2O3 © 2014 American Chemical Society

catalyst to produce n-alkanes, and then, n-alkanes undergo hydroisomerization to produce isoalkanes on a bifunctional metal/zeolite catalyst. Before hydroisomerization, the separation of n-alkanes and other byproducts is necessary. That is, the two-step process is complicated and costly. Herskowitz et al.9 proposes a one-step process combining deoxygenation and hydroisomerization on a bifunctional catalyst, which has many advantages such as a simplified process and reduced cost. The bifunctional catalysts usually consist of metals (Pt, Pd, and Ni) and acidic supports (such as SAPO-11 and HZSM-5). Recently, some researchers have reported noble metal bifunctional catalysts (such as Pt/SAPO-11 and Pd/SAPO-31) for producing high quality diesel-like alkanes in a one-step process.10−14 The noble metal bifunctional catalysts usually show good performance; however, the high cost may limit their application on a large scale. In some reports, the nickel-based bifunctional catalyst has been adopted in the one-step process.15−17 Liu et al.15 have found that Ni/SAPO-11 possesses good activity for the hydroconversion (involving Received: August 1, 2014 Revised: October 21, 2014 Published: October 22, 2014 7122

dx.doi.org/10.1021/ef501723p | Energy Fuels 2014, 28, 7122−7132

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SAPO-11 was cooled to room temperature and passivated in a 0.5% O2/N2 flow for 4 h. Ni/SAPO-11 was prepared according to the following procedure: SAPO-11 (60−100 mesh) was incipiently impregnated with an aqueous solution of Ni(NO3)2·6H2O, followed by drying at 120 °C for 12 h and calcination at 500 °C for 4 h. The Ni/SAPO-11 precursor was reduced at 450 °C for 1 h to prepare Ni/SAPO-11. The nominal Ni mass contents in Ni2P/SAPO-11 and Ni/SAPO-11 were 3%. 2.2. Catalyst Characterization. X-ray diffraction (XRD) patterns of catalysts were recorded on a D8 focus powder diffractometer operated at 40 kV and 40 mA with Cu Kα radiation (λ = 0.15406 nm). CO chemisorption was measured on a laboratory-made instrument. 200 mg passivated Ni2P/SAPO-11 or the Ni/SAPO-11 precursor was loaded to a quartz reactor (inner diameter 4 mm) and then reduced with a H2 flow (60 mL/min) at 450 °C for 1 h. Afterward, the sample was swept with a He flow (40 mL/min) at 450 °C for 1 h and then cooled to 30 °C. When the thermal conductivity detector (TCD) signal was stable, the pluses of CO were passed through the sample until the effluent areas of consecutive pulses were constant. The total dynamic CO uptake was calculated. NH3-TPD was also measured on the laboratory-made instrument. 100 mg passivated Ni2P/SAPO-11 or the Ni/SAPO-11 precursor was loaded to a quartz reactor (inner diameter 4 mm) and then reduced with a H2 flow (60 mL/min) at 450 °C for 1 h and then cooled to 100 °C. After NH3 adsorption for 30 min, the sample was swept with a He flow (60 mL/min) to remove the physically adsorbed NH3. NH3-TPD was performed in a He flow at a heating rate of 15 °C/min. The desorbed NH3 was detected by a TCD. Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800 instrument operated at 10 kV. The samples were placed on a conductive carbon tape and sprayed gold to avoid the electric charge accumulation. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F instrument. The powder samples were ultrasonically dispersed in ethanol and then deposited on a carbon film supported on a copper grill. The statistical number of particles is at least 250, and the mean diameter is calculated by the equation, dTEM = ∑(ni × di)/∑ni, where di and ni are the diameter and particle number, respectively. Thermogravimetric (TG) analysis of the catalyst was conducted on a Pyris 1 TGA in an air flow (100 mL/min) at a temperature ramp of 10 °C/min. Raman spectrum of the catalyst was obtained on a Thermo Fisher DXR Raman instrument with a laser of 532 nm. N2 adsorption−desorption isotherms were measured on a Micromeritics ASAP 2020 apparatus at −196 °C. The specific surface area was calculated by the BET equation. The micropore and external surface areas and the micropore volume were determined by the t-plot method. The meso-pore volume and diameter were determined by the BJH method from the desorption branch of the isotherm. The total pore volume (Vp) was estimated at relative pressure of 0.99. The mean pore diameter (d) was calculated using d = 4Vp/SBET. The Horvath− Kawazoe (HK) method was used to calculate the micropore diameter. 2.3. Reactivity Evaluation. The hydroconversion of methyl laurate was tested on a stainless-steel fixed-bed reactor (inner diameter of 12 mm). Before the reaction, the passivated catalyst or the Ni/ SAPO-11 precursor was reduced at 450 °C for 1 h in a hydrogen flow. Afterward, the temperature and H2 pressure were adjusted to the desired values and then the methyl laurate was fed into the reactor by a pump. The reactions were proved to be in the negligible mass- and heat-transfer limitations. The liquid products were identified by gas chromatograph (GC) standards and an Agilent GC6890-MS5973N. They were quantitatively analyzed on a SP-3420 GC with a FID and a HP-5 capillary column (30 m × 0.32 mm × 0.5 μm). n-Tridecane was used as an internal standard. The gaseous products were analyzed by using an online 102 gas chromatograph equipped with a TCD and a TDX-101 packed column. N2 was used as an internal standard to detect the amount of CO.

deoxygenation and hydroisomerization) of palm oil to produce isoalkanes. With regard to the supports, the properties (i.e., the acidity and pore structure) should be taken into account. Compared with the supports with strong acidity (such as HZSM-5 and HY), SAPO-11 shows better selectivity to isoalkanes in the HDO of methyl palmitate and soybean oil due to its moderate Brönsted acidity.16,17 In addition, the 10ring elliptical channel (0.39 nm × 0.63 nm in diameter) of SAPO-11 is suitable for the hydroisomerization of long nalkanes.18−20 Recently, transition metal (Ni, Co, Fe, Mo, and W) phosphides have been proved to be good for the deoxygenations of bio-oils as well as fatty acid esters.21−25 Among them, Ni2P has the best performance in the deoxygenation of methyl laurate as a model compound.25 Additionally, in contrast to metallic Ni, Ni2P had much lower activities for the C−C hydrogenolysis and methanation due to the ensemble and ligand effects of phosphorus.25−27 This is very positive in enhancing carbon yield and reducing H2 consumption. Particularly, our previous work28 has also shown that Ni2P/ SAPO-11 possesses better isomerization selectivity and lower cracking activity than Ni/SAPO-11 in the hydroisomerization of n-dodecane. Therefore, Ni2P-based bifunctional catalyst is potentially promising for the hydroconversion of fatty acid esters to isoalkanes. However, the hydroconversion of fatty acid ester are more complex because it combines the deoxygenation of fatty acid ester and the hydroisomerization of alkanes on the bifunctional catalyst. And, to our knowledge, no related research has been reported for the hydroconversion of fatty acid ester on the bifunctional Ni2P-based catalyst. Although we have investigated the performance of Ni2P/ SAPO-11 for the hydroconversion of methyl laurate, the high nickel loading (15 wt %) lead to few medium strength acid sites on the catalyst and subsequently scarce hydroisomerization.29 In the present work, Ni2P/SAPO-11 containing 3 wt % Ni, which showed a good performance in the hydroisomerizaiton of n-dodecane,28 was used to catalyze the hydroconversion of methyl laurate as a compound. In contrast to Ni2P, Ni has different structure and performance in hydrogenolysis and hydrodeoxygenation.25,30 However, the difference of the bifunctional Ni and Ni2P catalysts in the hydroconversion of methyl laurate is not clear. For comparison, the performance of the bifunctional Ni/SAPO-11 was also tested. We investigated the influence of reaction conditions (including temperature, WHSV, and H2 pressure) on the catalyst performance, tested the catalyst stability, and analyzed the catalyst deactivation. The result may provide valuable information for developing Ni2Pbased catalyst to produce diesel-like hydrocarbons with good low-temperature performance.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The SAPO-11 molecular sieve was purchased from The Catalyst Plant of Nankai University, China. According to the procedure in the previous work,28 Ni2P/SAPO-11 catalyst was prepared by the temperature-programmed reduction (TPR) method. First, SAPO-11 (60−100 mesh) was incipiently impregnated with an aqueous solution of Ni(NO3)2·6H2O and NH4H2PO4, where the Ni/P molar ratio was 1.0. After drying at 120 °C for 12 h and calcination at 500 °C in air for 4 h, the SAPO-11supported nickel phosphate precursor was prepared. Second, the precursor was reduced by H2 from room temperature to 250 °C at a rate of 10 °C/min, and then from 250 to 650 °C at a rate of 1 °C/min and held at 650 °C for 3 h. After the reduction, the prepared Ni2P/ 7123


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The conversion (X) of methyl laurate and the selectivity to product i (Si) were defined as follows:

Scheme 2. Classical Reaction Pathway for the Formation of Isomers and Cracking Products over the Bifunctional Catalyst

⎛ n⎞ X = ⎜1 − ⎟ × 100% n0 ⎠ ⎝ Si =

ni × 100% n0 − n

where n0 and n are the moles of methyl laurate in the feed and product, respectively; ni denotes the mole of product i (e.g., n-undecane, ndodecane, isoundecane, and isododecane).

3. RESULTS AND DISCUSSION The fresh Ni/SAPO-11 and Ni2P/SAPO-11 catalysts have been characterized in the previous work.28 The XRD patterns due to the P−OH groups, while the medium strength acid sites are dominatingly the bridged hydroxyl group of Si(OH)Al. When Ni or Ni2P was supported on SAPO-11, the acid sites (the P−OH groups and the bridged hydroxyl groups of Si(OH) Al on SAPO-11) still maintained, but some of them could be covered by Ni or Ni2P particles. In addition, some new weak sites due to the acidity of Ni2P were created on Ni2P/SAPO11.28 As indicated in Figure 1, Ni/SAPO-11 had more medium strength Brönsted acid sites but less weak strength acid sites than Ni2P/SAPO-11. In other words, there is less medium strength Brönsted acid sites on Ni2P/SAPO-11, which is due to the coverage of acid sites with P. Even so, both Ni/SAPO-11 and Ni2P/SAPO-11 were of bifunctional. In the hydroconversion of methyl laurate, they will catalyze not only deoxygenation but also hydroisomerization, which is demonstrated in the present work. Here, we first investigated the effect of the reaction conditions on the reactivities of Ni/SAPO-11 and Ni2P/SAPO-11. 3.1. Influence of Reaction Conditions on Catalyst Reactivity. In the hydroconversion of methyl laurate on Ni/ SAPO-11 and Ni2P/SAPO-11, C11 and C12 hydrocarbons were detected as main products, wherein isoundecane (i-C11) and isododecane (i-C12) were expected. C1−C5 and C6−C10 hydrocarbons (including isoalkanes) were detected in the gas and liquid products, respectively. The C2−C10 hydrocarbons were derived from the cracking of C11 and C12 hydrocarbons. Apart from the cracking, the hydrogenolysis of O−CH3 bond also led to the formation of methane. In addition, there were some oxygen-containing intermediates (dodecanoic acid, dodecanal, and dodecanol) in the liquid product. The product distribution on Ni2P/SAPO-11 was very different from those on Ni2P/SiO2, Ni2P/MCM-41, and Ni2P/SBA-15 where n-

Figure 1. NH3-TPD profiles of (a) SAPO-11, (b) Ni/SAPO-11, and (c) Ni2P/SAPO-11.

(Figure 1S in Supporting Information) shows that the Ni or Ni2P loading did not affect the structure of SAPO-11 zeolite, and Ni and Ni2P highly dispersed on SAPO-11 due to low nickel loading (3 wt %). Here, the CO uptakes of Ni/SPAO-11 and Ni2P/SAPO-11 were measured as 4 and 2 μmol/g, respectively. The lower CO uptake of Ni2P/SPAO-11 is related to the introduction of phosphorus and the higher preparation temperature. It is demonstrated that there are the weak and medium strength Brönsted acid sites on SAPO-11,31,32 which correspond to the NH3 desorptions with the peaks at about 210 and 320 °C (Figure 1), respectively. The weak acid sites are mainly

Scheme 1. Proposed Reaction Pathway for Deoxygenation of Methyl Laurate

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Figure 4. Effect of pressure on reactivity of Ni2P/SAPO-11.

Figure 2. Effect of temperature on reactivities of Ni/SAPO-11 and Ni2P/SAPO-11.

Figure 5. Performance of Ni/SAPO-11 and Ni2P/SAPO-11 with time on stream. Figure 3. Effect of WHSV on reactivity of Ni2P/SAPO-11.

the same as that in Ni2P/SiO2, Ni2P/MCM-41, and Ni2P/SBA15. Based on previous works25,33 and the product distribution in the present work, a modified reaction pathway for hydroconversion of methyl laurate on Ni/SAPO-11 and Ni2P/SAPO11 is proposed in Scheme 1. During the hydroconversion, the

dodecane and n-undecane were dominating products, and the amounts of the cracked hydrocarbons and the isomers were scarce.33 This is due to only weak Brönsted strength acid sites (i.e., the P−OH groups) on Ni2P/SiO2, Ni2P/MCM-41, and Ni2P/SBA-15. Regardless, the role of Ni2P in Ni2P/SAPO-11 is 7125


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the hydroconversion of methyl laurate, alkene may not only be formed via the dehydrogenation of n-alkane but also via the decarbonylation of methyl laurate or its intermediates (such as dodecanoic acid and dodecanal) as well as the dehydration of dodecanol (Scheme 1). The latter case may be more dominating because the dehydrogenation of n-alkanes needs metal sites and subsequently competes with the deoxygenation for metal sites. The alkenes produced via decarbonylation or dehydration are converted to isoalkanes as the mechanism same to the typical one of n-alkane hydroisomerization. In addition, the carbenium intermediates undergo cracking reaction to cracked products. Also, the cracked products were produced from the hydrogenolysis of alkanes on Ni sites.25,28 3.1.1. Temperature. The effect of temperature on the reactivities of Ni/SAPO-11 and Ni2P/SAPO-11 is shown in Figure 2. As the temperature increased from 320 to 380 °C, the conversion of the methyl laurate increased from 59.1% to 99.3% on Ni2P/SAPO-11 and from 95.5% to 99.0% on Ni/ SAPO-11. To reveal the intrinsic activities of the catalysts, the TOFs based on the CO uptakes were calculated when the conversion of methyl laurate was controlled below 50% (indicated in the Supporting Information). At 360 °C, the TOFs on Ni2P/ SAPO-11 and Ni/SAPO-11 were about 13.1 and 22.3 s−1, respectively. That is, Ni/SAPO-11 had a higher intrinsic activity than Ni2P/SAPO-11. As indicated in Figure 2, the total selectivity to the C11 and C12 hydrocarbons (SC11+C12) first increased and then decreased. SC11+C12 exceeded 97% at 340 and 360 °C on Ni2P/SAPO-11, while it decreased from 97.2% to 83.2% on Ni/ SAPO-11 with increasing temperature from 320 to 380 °C. The high temperature promoted the cracking reaction. Indeed, the selectivity to C6−C10 hydrocarbons (SC6−C10) increased from 0.9% to 9.8% on Ni/SAPO-11 and from 0.7 to 4.3% on Ni2P/ SAPO-11. Clearly, Ni2P/SAPO-11 had lower activity than Ni/ SAPO-11 for the C−C hydrogenolysis, which was also found in the hydrogenolysis of glycerol and the hydroisomerization of ndodecane.28,30 We suggest that this result is mainly ascribed to the ligand (i.e., stabilization of the Ni 3d levels and Ni → P light charge transfer) and ensemble (i.e., reduction in the number of exposed Ni sites) effects of P.26 In addition, the total selectivity to oxygen-containing intermediates (Soxy) decreased on Ni2P/SAPO-11 and Ni/SAPO-11 with the temperature increasing, but it was always less than 0.6%. This means that the increase of reaction temperature promoted the deoxygenation reaction. As the temperature increased from 320 to 380 °C, the selectivity to i-C11 and i-C12 (Si‑C11+i‑C12) increased from 2.3% to 14.9% on Ni2P/SAPO-11 and from 5.2% to 39.4% on Ni/ SAPO-11. That is, increasing temperature promoted the hydroisomerization. The lower Si‑C11+i‑C12 on Ni2P/SAPO-11 than that of Ni/SAPO-11 is mainly due to less medium strength Brönsted acid sites on Ni2P/SAPO-11. The molar ratio between C11 and C12 hydrocarbons (C11/ C12 ratio) reflects the selectivity between the decarbonylation and HDO pathways. With increasing temperature from 320 to 380 °C, the C11/C12 ratio increased from 3.8 to 18.9 on Ni2P/ SAPO-11 and from 0.7 to 3.6 on Ni/SAPO-11. Clearly, the increase of temperature favored the decarbonylation reaction.37 This is reasonable because the decarbonylation reaction is endothermic. Interestingly, Ni2P/SAPO-11 gave a higher activity for decarbonylation than Ni/SAPO-11. This is opposite to the case for Ni/SiO2 and Ni2P/SiO2. The previous work

Figure 6. Performance of Ni2P/SAPO-11 with time on stream.

Figure 7. XRD patterns of (a) fresh Ni/SAPO-11; (b) used Ni/SAPO11; (c) fresh Ni2P/SAPO-11; (d) used Ni2P/SAPO-11; (e) used Ni2P/SAPO-11*.

C11 and C12 hydrocarbons are formed via decarbonylation and HDO pathways, respectively. HDO pathway contains successive reactions (such as hydrogenolysis, hydrolysis, hydrogenation and dehydration), i.e., methyl laurate → dodecanoic acid → dodecanal → dodecanol → C12 hydrocarbons. Because no CO2 was detected, decarboxylation did not occur on Ni/ SAPO-11 and Ni2P/SAPO-11. The deoxygenation mainly occurs on the metal sites25,34 and also on the Brönsted acid sites to some extent, while hydroisomerization requires both the metal and the acid (especially the medium strength Brönsted type) sites.35 Herein, it is worth noticing that the formation of isoalkane may not completely undergo the same mechanism as the hydroisomerization of n-alkane. Scheme 2 depicts the typical mechanism of n-alkane hydroisomerization.36 However, during 7126


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Figure 8. TEM images and diameter distribution of the catalysts (a) fresh Ni/SAPO-11; (b) used Ni/SAPO-11; (c) fresh Ni2P/SAPO-11; (d) used Ni2P/SAPO-11; (e) used Ni2P/SAPO-11*.

decarbonylation pathway. In addition, the medium strength Brönsted acid site easily releases the H+ ion with electrophilicity, which can hydrogenate the CO group to produce the HDO product (i.e., C12).16,29 Compared with those on Ni2P/SAPO-11, more medium strength Brönsted acid sites on Ni/SAPO-11 can account for its higher selectivity to HDO pathway (i.e., lower C11/C12 ratio). That is, there was a synergism between the metal site and the medium strength Brönsted acid site on Ni/SAPO-11.

indicates that Ni2P/SiO2 had much a lower decarbonylation activity than Ni/SiO2.25 Since SiO2 is relatively inert, the properties of Ni and Ni2P determine the deoxygenation pathways. We have proposed that the deoxygenation pathway is mainly determined by the metallic property (i.e., the electron density) and Brönsted acidity.25,29 The high electron density of the Ni site and the weak Brönsted acid site may favor the decarbonylation.14,25,29,33 Compared with Ni2P/SiO2, the higher electron density of the Ni site on Ni/SiO2 favors the 7127


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Figure 9. SEM images of (a) fresh Ni/SAPO-11; (b) used Ni/SAPO-11; (c) fresh Ni2P/SAPO-11; (d) used Ni2P/SAPO-11; (e) used Ni2P/SAPO11*.

In addition, as the temperature increased from 320 to 380 °C, the CO/C11 ratio was always about 1.0 on Ni2P/SAPO-11, while it decreased from 0.95 to 0.68 on Ni/SAPO-11. Because the C11 hydrocarbons are derived from decarbonylation, the CO/C11 ratio of about 1.0 indicates that the CO methanation did not occur on Ni2P/SAPO-11. This is consistent with the previous result on Ni2P/SiO2.25 However, Ni/SAPO-11 had higher activity for CO methanation, which was also promoted with increasing temperature. The less methanation activity of Ni2P/SAPO-11 is ascribed to the lower electron density of Ni due to the ligand effect of P.25 Here, we notice that the effect of reaction temperature on the selectivity to isoalkane (i.e., i-C11 and i-C12) in the hydroconversion of methyl laurate is different from that in the hydroisomerization of n-dodecane (reported in ref 28). As the reaction temperature increased, the selectivity to isoalkane (i-C11 and i-C12) increased in the hydroconversion of methyl laurate; however, the selectivity to i-C12 decreased in the

Figure 10. TG-DTG curves of the fresh and used catalysts. 7128


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hydroisomerization and cracking reaction). Also, the reduced C11/C12 ratio indicates that the HDO pathway was promoted with increasing WHSV. Even so, the C11/C12 ratio larger than 6.4 indicates that the decarbonylation reaction was still primary. In all, WHSV had a great impact on Si‑C11+i‑C12 and SC6−C10. Unfortunately, the increase of Si‑C11+i‑C12 was accompanied by that of SC6−C10. 3.1.3. H2 Pressure. Figure 4 shows the performance of Ni2P/ SAPO-11 under different H2 pressures. As the pressure increased from 1 to 5 MPa, both the conversion of methyl laurate and SC11+C12 exceeded 96%. However, Si‑C11+i‑C12 and SC6−C10 decreased, which might be ascribed to the competitive adsorption of H2. The C11/C12 ratio decreased obviously. It is reasonable that increasing H2 pressure favored the HDO pathway. Similar results have been found by Sotelo-Boyás.38 Thus, the H2 pressure could influence the pathway of deoxygenation but had little effect on the activity of Ni2P/ SAPO-11. 3.2. Stability of Ni/SAPO-11 and Ni2P/SAPO-11. Figure 5 shows the stability of Ni/SPAO-11 and Ni2P/SAPO-11 at 360 °C, 3.0 MPa, WHSV of 8 h−1, and an H2/methyl laurate molar ratio of 25. After the reaction for 101 h, the conversion (∼95%) and SC11+C12 (∼96%) on Ni2P/SAPO-11 did not obviously change, and Soxy was still less than 0.6% at 101 h. In contrast, the conversion on Ni/SAPO-11 stabilized at 99% before 61 h and dropped to 71% at 101 h, and SC11+C12 increased from the initial 74.7% to 96% at 85 h and then decreased sharply to 84% at 101 h. Meanwhile, Soxy was less 0.1% before 61 h, and then increased to 8.3% at 101 h. Clearly, Ni2P/SAPO-11 was more stable than Ni/SAPO-11. Si‑C11+i‑C12 and SC6−C10 decreased remarkably on both Ni2P/SAPO-11 and Ni/SAPO-11 with time on stream, especially Si‑C11+i‑C12. After reaction for 101 h, Si‑C11+i‑C12 decreased from 6.4% to 0.5% on Ni2P/SAPO-11 and from 46% to 5.8% on Ni/SAPO-11. The lower Si‑C11+i‑C12 on Ni2P/SAPO-11 is attributable to its less medium strength Brönsted acid sites. In addition, the C11/C12 ratio increased from 6.4 to 14.1 on Ni2P/SAPO-11, while it maintained at about 1.2 before 85 h and then slightly increased to 2.1 at 101 h on Ni/SAPO-11. In short, compared with Ni/ SAPO-11, Ni2P/SPAO-11 showed superior stability but lower Si‑C11+i‑C12. As mentioned in section 3.1.2, the decrease of WHSV favored Si‑C11+i‑C12. Ni2P/SAPO-11 was also tested at WHSV of 2 h−1 (see Figure 6). As depicted in Figure 6, the conversion of methyl laurate was close to 100% during 101 h. SC11+C12 increased from 77% to 88% before 73 h and then did not

Figure 11. Raman spectra of the spent catalysts: (a) used Ni2P/SAPO11* ; (b) used Ni2P/SAPO-11; (c) used Ni/SAPO-11.

hydroisomerization of n-dodecane.28 This difference is probably related to the different mechanisms. Different from the hydroisomerzation of n-dodecane, the hydroconversion of methyl laurate included deoxygenation and subsequent hydroisomerization, i.e., the C11 and C12 hydrocarbons (including alkene and n-alkane) were first formed and then isomerized to i-C11 and i-C12. As the reaction temperature increased, the conversion and SC11+C12 increased, i.e., the concentration of C11 and C12 hydrocarbons increased, which is favorable for the hydroisomerization. Although Ni/SAPO-11 gave higher Si‑C11+i‑C12 than Ni2P/ SAPO-11, it had higher activities for cracking and methanation. This leads to a lower yield of C11 and C12 hydrocarbons and a larger H2 consumption. To increase Si‑C11+i‑C12 on Ni2P/SAPO11, the effects of WHSV and H2 pressure on the performance of Ni2P/SAPO-11 were also investigated. 360 °C was selected as the reaction temperature in the following study. 3.1.2. WHSV. Figure 3 shows the reactivity of Ni2P/SAPO11 at different WHSVs of methyl laurate. As WHSV increased from 2 to 8 h−1, the conversion of methyl laurate slightly decreased but still was maintained above 95%. SC11+C12 increased from 76% to 96%, while SC6−C10 decreased from 18.4% to 3.1% and Soxy increased from 0.01% to 0.42%. In addition, Si‑C11+i‑C12 decreased from 37.7% to 6.4%. The increase of WHSV corresponds to the reduction of contact time, which restrains the secondary reactions (such as

Table 1. Textural Properties of SAPO-11, Ni/SAPO-11, and Ni2P/SAPO-11 Catalysts before and after Reaction sample SAPO-11a Ni/SAPO11a Ni2P/SAPO11a used Ni/ SAPO-11 used Ni2P/ SAPO-11 used Ni2P/ SAPO-11* a

SBET (m2/g)

t-plot micropore area (m2/g)

t-plot external surface area (m2/g)

pore volume (cm3/g)

t-plot micropore volume (cm3/g)

mesopore volume and diameter (cm3/g; nm)

micropore width (nm)

average pore diameter (nm)

166 121

122 80

44 40

0.21 0.18

0.064 0.042

0.14 12.2 0.13 13.8

0.60 0.57

5.0 5.8

113

79

33

0.14

0.041

0.10 12.1

0.60

5.1

28

0

28

0.11

0

0.11 15.9

-

15.9

26

1.2

25

0.11

0.001

0.10 19.0

-

16.7

37

9.7

28

0.11

0.005

0.10 14.9

-

11.8

Data from ref 28. 7129


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below 100 °C is primarily ascribed to the desorption of water. For the used catalysts, the weight loss below 200 °C is related to the desorptions of adsorbed water and organics. During 230−750 °C, the weight loss is attributed to the combustion of carbonaceous deposit, and two weight loss steps imply that there were two types of carbonaceous deposits with different activities. The amounts of carbonaceous deposit on the used Ni/SAPO-11, Ni2P/SAPO-11, and Ni2P/SAPO-11* catalysts were 3.6%, 4.3%, and 4.0%, respectively. Ni2P/SAPO-11 had slightly larger amount of carbonaceous deposit than Ni/SAPO11. Raman spectroscopy is a powerful technique to characterize the structure of carbonaceous deposit. Figure 11 shows the Raman spectra of the used catalysts in range of 1300−1800 cm−1. The broad band between 1300 and 1480 cm−1, known as D band, is related to the vibrations of carbon atoms with dangling bonds in the disordered graphite planes,41 while the band at 1591 cm−1, known as G band, is attributed to the stretching mode of carbon sp2 bonds of the typical graphite.42 The typical graphite is more unreactive than the disordered one.43 In addition, the broad band indicates that the carbonaceous deposit is highly inhomogeneous in nature.44 The area ratio between G and D bands (AG/AD ratio) is used to present the relative amount of the two kinds of graphite. The AG/AD ratios were 2.4, 2.1, and 2.2 on the used Ni/SAPO-11, Ni2P/SAPO-11, and Ni2P/SAPO-11* catalysts, respectively. The close AG/AD ratios indicate that the type and nature of carbonaceous deposit on the used Ni/SAPO-11 and Ni2P/ SAPO-11 were similar. 3.3.3. Textural Properties. Table 1 shows the surface areas and pore structures of the fresh and used catalysts. In comparison with those of the fresh catalysts, the used catalysts showed much lower BET surface areas, which is mainly due to the loss of micropore areas. Clearly, there were no micropores measured for the used Ni/SAPO-11, indicating the complete blockage of micropores with carbonaceous deposit that might grow at the pore mouth of SAPO-11. 2% and 12% of the micropore areas still maintained for the used Ni2P/SAPO-11 and Ni2P/SPAO-11*, respectively. This indicates that carbonaceous deposit did not block the micropores completely, and more micropore areas were retained at lower WHSV for testing Ni2P/SPAO-11. The average pore diameters became obviously larger after the reaction mostly due to the loss of micropores. Indeed, they were close to the mesopore diameters. In addition, we suggest that the changes of surface area and pore structure had less association with the sintering of SAPO-11 because there was no obvious difference in the morphology and size of SAPO-11 particles before and after reaction. As above-mentioned, the carbonaceous deposit formed on both used Ni/SAPO-11 and Ni2P/SAPO-11 during the reaction, which led to the blockage of micropores of SAPO11. The sintering of Ni particles took place on Ni/SAPO-11, while that of Ni2P particles was not obvious on Ni2P/SAPO-11. Thus, we propose that the Ni/SAPO-11 deactivation is mainly due to the sintering of Ni particles and the carbonaceous deposit, while the Ni2P/SAPO-11 deactivation is mostly ascribed to the carbonaceous deposit. Although the carbonaceous deposit could cover both Ni and acid sites, it might preferentially cover the acid sites. In other words, although the carbonaceous deposit could be formed on Ni sites via dehydrogenation, its formation was mainly catalyzed by acid sites via polymerization. This is reasonable because the conversion of methyl laurate did not decrease obviously but

change obviously. This corresponds to the decrease of SC6−C10. Si‑C11+i‑C12 maintained above 35% before 37 h and then decreased to 28.6% at 101 h. Clearly, Si‑C11+i‑C12 was greatly enhanced at WHSV of 2 h−1. In addition, the C11/C12 ratio was increased from 7.1 to 22.2. 3.3. Analysis of Catalyst Deactivation. As mentioned above, from the perspective of the conversion, Ni2P/SAPO-11 was very stable and much superior to Ni/SAPO-11. However, from the view of the selectivity to isoalkanes that decreased with time on stream, both Ni2P/SAPO-11 and Ni/SAPO-11 were deactivated. To analyze these reasons, the fresh and used Ni/SAPO-11 and Ni2P/SAPO-11 catalysts were characterized. The used Ni2P/SAPO-11 and Ni/SAPO-11 are the samples after reaction for 101 h at WHSV of 8 h−1. And, the used Ni2P/ SAPO-11* represents the sample after reaction for 101 h at WHSV of 2 h−1. Usually, the catalyst deactivation is attributed to sintering, carbonaceous deposit, and poisoning. In the present work, we speculate that the catalyst deactivation is mainly related to sintering and carbonaceous deposit. 3.3.1. Sintering. Figure 7 shows the XRD patterns of the fresh and used catalysts. It can be found that the structure of SAPO-11 was not destroyed after reaction. For all catalysts, no obvious peaks due to metallic Ni or Ni2P were observed due to the low nickel loading (3%). To obtain the sizes of Ni or Ni2P particles, the samples were also characterized by TEM. Figure 8 displays the TEM images of the fresh and used catalysts. The formation of metallic Ni phase and Ni2P phase were verified by the enlarged TEM images. The Ni or Ni2P particles were dominatingly larger than 2 nm, while SAPO-11 possesses nonintersecting elliptical 10-membered ring pores (0.39 nm × 0.63 nm in diameter). Therefore, Ni or Ni2P particles were mostly dispersed on the SAPO-11 particle surface (including in the pore mouth). For the Ni2P/SAPO-11 samples before and after reaction, the d-spacing values of 0.221 and 0.203 nm are corresponding to the (111) and (201) crystallographic planes of Ni2P, respectively. As shown in Figure 8a and b, the Ni particles agglomerated from 3.8 to 4.8 nm after the reaction. That is, the sintering of Ni particles took place during the reaction. Figure 8c−e shows that the Ni2P particle size (about 8 nm) did not obviously change after the reaction. In other words, the Ni2P particle possessed high resistance to sintering. The sintering of the Ni particles is probably related to the formation of water during the hydroconversion. As indicated in Figure 2, the HDO pathway was more favorable on Ni/SAPO11 than on Ni2P/SAPO-11. This means that there was more amount of water formed on Ni/SAPO-11, while water could promote the sintering of the Ni particles.39 However, there was a less amount of water formed on Ni2P/SAPO-11. Moreover, as indicated by Oyama et al.23 and our previous work,40 the extra phosphorus as well as the phosphorus species in Ni2P could interact in a preferential way with water molecules, subsequently protecting Ni sites from oxidation and also probably Ni2P particles from sintering. In other words, the deactivation of Ni/SAPO-11 might also be related to the oxidation of surface Ni due to water.27 The fresh and used catalysts were also characterized by SEM. As indicated by Figure 9, the morphology and size of SAPO-11 particles did not obviously change after the reaction for 101 h under the present condition. 3.3.2. Carbonaceous Deposit. Figure 10 shows the TGDTG curves of the fresh and used catalysts. For the TG curves of the fresh Ni/SAPO-11 and Ni2P/SAPO-11, the weight loss 7130


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Si‑C11+i‑C12 decreased obviously for Ni2P/SAPO-11 during 101 h. As indicated by the previous work,25 the conversion of methyl laurate mainly took place on metal ones. However, hydroisomerization needs both metal and acid sites. For Ni/ SAPO-11, the sintering of Ni particles not only reduced the conversion of methyl laurate but also restrained the hydroisomerization.

4. CONCLUSIONS We present the Ni2P/SPAO-11 catalyst for the hydroconversion of methyl laurate as a model compound to hydrocarbons. Here, we found that the reaction conditions (especially temperature and WHSV) affected the catalyst activity, deoxygenation pathway, and the selectivity to the isoalkanes on Ni/SAPO-11 and Ni2P/SPAO-11. On the whole, the increase of temperature and the deceases of WHSV and H2 pressure favored the conversion of methyl laurate while promoting the decarbonylation and hydroisomerization as well as cracking reactions. In the stability test, Ni/SAPO-11 suffered from serious deactivation for both deoxygenation and hydroisomerization due to the sintering of the Ni particles and carbonaceous deposit, while Ni2P/SAPO-11 only showed the deactivation for hydroisomerization because of the coverage of acid sites with carbonaceous deposit. In addition, water formed during the hydroconversion is harmful for the catalyst stability. At 360 °C, 3.0 MPa, WHSV of 2 h−1, and H2/methyl laurate molar ratio of 25, the conversion were always close to 100% on Ni2P/SAPO-11 during 101 h, while the total selectivity to isoundecane and isododecane decreased from 36.9% to 28.6%. Even so, Ni2P-based bifunctional catalyst is very promising to produce high quality liquid fuels. Improving its activity for hydroisomerization is an area for further exploration in the future.

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ASSOCIATED CONTENT

S Supporting Information *

Figure S1 and details on methods to demonstrate if there were mass- and heat-transfer limitations and calculating the TOF. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-22-27890865. Fax: +86-22-87894301. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 21176177), the Natural Science Foundation of Tianjin (No. 12JCYBJC13200), State Key Laboratory of Catalytic Materials and Reaction Engineering (RIPP, SINOPEC), and the Program of Introducing Talents to the University Disciplines (B06006).

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REFERENCES

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Hydroconversion of Methyl Laurate as a Model Compound to

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